The present invention relates to the structure of a sensor, for example, an air-fuel ratio sensor for detecting the concentration of oxygen density using as a detecting element a ceramic solid electrolyte.
To construct a sensor for detecting conditions of the ambient atmosphere and producing an electrical signal indicative of the detected parameters, for example, an air-fuel ratio sensor, a structure as shown in a partial cutaway view of FIG. 1 has been proposed. FIG. 2 is a perspective view showing a ceramic sleeve 54a used in the sensor structure of FIG. 1, and FIG. 3 is a perspective view showing a ceramic guide 50ax used in the same sensor structure.
A ceramic detecting element 14a fixed to ceramic guides 50ax and 50ay through cement 52a is held in the ceramic sleeve 60a through which lead wires 2 pass in an insulated state. Seal members 19a and 20a are provided on the side of a base portion 17ay of the ceramic sleeve 54a. After the space therebetween is filled with talc powder 58a, the ceramic sleeve 54a and a cylindrical main metal tubular member 24a are heat-sealed to fix them together. At the same time, a metal outer cylinder 28a, disposed against the outside of the ceramic sleeve 54a and on the side of the base portion 17ay on which a flange portion of the ceramic sleeve 54a is provided, is simultaneously heat-sealed. A protective outer cylinder 32a externally touching the metal outer cylinder 28a is sealed by heating along six directions.
In this proposed sensor 4a structure, however, the ceramic detecting element 14a is fixed in the ceramic sleeve 54a through the glass 18a and cement 52a, 56ax, and 56ay. Accordingly, problems arise in that splits or cracks often occur in the ceramic sleeve 54a when the ceramic sleeve 54a is externally sealed, thereby giving rise to difficulties relating to the sealing property and allowing for the possibility of damage in the internal structure of the sensor 4a.